The polymer manufacturing industry, and particularly the polyvinyl chloride industry, which produces most of its products by the suspension polymerization of monomeric materials, has long been plagued by the need to prevent undesirable polymer buildup on the interior surfaces of reaction vessels. This polymer buildup is of such a nature as to require frequent cleaning necessitating a complete shutdown of the polymerization reactors. The polymer buildup, if not removed, will slough off the reactor walls into the reaction mixture, causing "gels" in the polymeric product. Polymer buildup removal procedures cost the industry millions of dollars yearly, and have been calculated to reduce the industry's commercial production capacity by up to 15 percent. In addition to cost and downtime the industry is faced with the problem of containment of potentially toxic monomeric materials resulting from repeated opening of reactors for cleaning purposes. Consequently, the polymer manufacturing industry is very interested in reducing or eliminating the polymer buildup problem.
Various treatments have been proposed to combat such polymer buildup. U.S. Pat. No. 3,825,434 teaches the use of hydrophilic films on the internal surface of polymerization reactors. Polyvinyl alcohol, gelatin, methyl cellulose, carboxymethyl cellulose, polyethylene oxide, polyvinyl pyrrolidone, polyacrylic acid and other polyelectrolytes such as polymethacrylic acid, polyethacrylic acid, polychloroacrylic acid, polycrotonic acid and polycyanoacrylic acid are named in the process of the above-specified patent. U.S. Pat. No. 3,575,079 teaches the treatment of reactor surfaces with an aqueous solution of one or more water-soluble derivatives certain amino carboxylic acids selected from the group consisting of mono- and polycarboxylic acids. U.S. Pat. No. 3,849,179 employs coatings of polyethylenimine to retard polymer buildup but teaches the need for a very clean surface prior to application of such coating In what seems to be direct contradiction to the above teaching for use of hydrophilic surfaces, U.S. Pat. No. 3,778,423 discloses the use of alpha-methylstyrene as a reactor coating material to inhibit buildup in polyvinyl chloride suspension polymerization processes. Such treatment is economically undesirable, however, and is not as effective as the treatment contemplated by the present invention. More particularly, the gas phase treatment of the present invention provides a more uniform contact with the interior surfaces of the reactor.
Accompanying the problem of polymer buildup on the walls of the reactor is the additional complication which centers around the tendency of the reactor contents to foam during removal of oxygen from the aqueous portion of the reaction mixture prior to introduction of monomer.
Notwithstanding the fact that prior known processes render improvements in the prevention of polymer buildup, such processes do not resolve the problem of foaming of reactor contents during oxygen purge of the reactor prior to subsequent polymerization reactions nor do they provide suitable means for treatment of an uncleaned polymerization reactor which treatment enables substantially uninterrupted continuation of commercial production.
Accordingly, it is an object of this invention to provide a process for eliminating or retarding the buildup of polymeric material on reactor walls during polymerization of substantially water-insoluble ethylenically unsaturated monomer.
Another object is to provide a means for effectively affixing a hydrophilic coating onto the reactor walls without the necessity of carefully applied primer coats and/or the use of thermal curing steps for such hydrophilic coatings.
It is another object of the present invention to reduce the tendency of the reactor contents to foam during oxygen purge of the reactor prior to subsequent polymerization reactions.